[Wallee]

Good afternoon everyone. Once again, I am looking for help from the "expert" yorkie owners. I have a pretty curly Biewer Yorkie (Wallee) and struggling to keep him groomed. I once had an American Cocker Spaniel whom I groomed myself (and for those of you who know the breed, not an easy task) so assumed I would be able to handle this little guy.
My biggest problem is getting the knots out from under his arms. I have a grooming table but cannot get him to lay on that to groom this area. He flips over and gets playfully nippy. I am able to manage this area a bit on the couch when I have him up against the back of the couch but again after a period of time, he starts to get roudy and fidgety.
Can anyone suggest the best tools to get the knots out from under his arm? I am presently trying to use two different brushes, (one of them being a slicker which he seems to go crazy from the sound it makes) and the other one is basically a rubber pin brush. I also have some spray that I spray him with before brushing however it doesn't seem to help the matts close to the skin.
Is it possible for this guy to be able to keep a long coat with having the cottony curly coat or will I be better off just to keep his hair cut in puppy cut?
Any grooming advice will be appreciated.

BTW, he is just 6 months old and I have been doing this since I got him at 12 weeks. He is great at bathing, blow drying and even brushing his teeth, but this brushing is a real struggle.